Conventionally, the path of a tunnel in tunnel construction is surveyed in the following manner using a position locating instrument. First, a collimated light beam from a laser source of a master device placed on a shaft at the entrance of a tunnel is applied to an intermediate prism unit of a slave device placed inside the tunnel. The intermediate prism unit of the slave device has two wedge prisms installed therein. The position locating instrument changes the vertical and horizontal travel directions of the light beam from the master device by appropriately rotating the two wedge prisms of the intermediate prism unit, and thereby applies the light beam to an intermediate prism unit of the next slave device. Whether the light is applied to the intermediate prism unit can be determine d by measurement of a signal from a photodetector installed near the paired wedge prisms through which light should pass.
Multiple slave devices are arranged in a line inside the tunnel, and the light beam travels along the curved path of the tunnel by being deflected sequentially by the intermediate prism units of the slave devices, and reaches the slave device located at a final target point, which is the head of the tunnel. Hereinafter, for the illustrative convenience, a side close to the master device is called an upper side, and a side away from the master device is called a lower side. After the light beam reaches the slave device at the lowermost side, the path of the tunnel can be obtained based on the angles of rotation of the paired wedge prisms of the intermediate prism units of all the slave devices.
This position locating method in which the slave devices arranged in a line in a tunnel emit and receive a light beam along the tunnel needs to find six parameters: the position of a lower-side slave device relative to the master device (a vertical angle, a horizontal angle, and the distance between the slave devices) and the attitude (pitch, roll, and yaw) represented as angles formed between the optical axis of a light beam and reference lines of the position locating instrument. However, the conventional method using the wedge prisms can find only two values: the vertical angle and the horizontal angle. Of the rest of the parameters, the distance, the pitch, and the roll are measured using other position locating methods. This makes the tunnel survey time- and labor-consuming and inefficient. In addition, there is no means for measuring the yaw, which can be a factor in generating error when the slave devices are increased in number.
The present invention has an object to achieve a position locating instrument for a position locating method in which slave devices arranged in a line emit and receive a light beam, the position locating instrument reducing error factors, increasing survey accuracy, and improving survey efficiency by providing all or some of necessary position and attitude parameters.